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Full text of "The Cambridge Natural History"

3 so

THE SUSU

Tnrsiops. It has no dorsal fin, the teeth are small and numerous
(forty-four), and the pterygoids are separate. There are two
species, T. borealis and T. peronii, the former being northern and
the latter more widely spread.
The genus Cephcdorhynchus has for its chief characters the
following:—Teeth twenty-five to thirty-one, small and sharp,
Pterygoids widely separated. Dorsal fin not falcate, but tri-
angular or ovate in form. Beak not well marked off from the
head. The species of this genus are all southern in range; four
are perhaps to be allowed.
Fam. 3. Platanistidae.—This family of Odontocetes may be
listinguished from the Dolphins by the following assemblage of
structural features:—Cervical vertebrae all free, and each one of
some length (for a Cetacean). Jaws long and narrow, with a
sonsiderable length of symphysis. Teeth very numerous.
This very meagre series of differential characters is largely due
;o   Pontoporia,   on   the   Platanistid   side,   and  to   Monodon   and
Delphivuxpterus upon the Delphinid side.     Otherwise the family
?lafcanistidae would be extremely distinct.     The two last-named
genera have separate cervical vertebrae, and in the Beluga at any
ute   this   is   expressed   externally   by   a    quite   distinct   neck.
Moreover, as Mr. True has pointed out, the pterygoid bones have
iot    the    involuted   cavity   below   which    characterises    other
Dolphins;  and  they have, what   other Dolphins  have  not, an
xticulation. outwards with the roofing bones of the skull.     Sir
/V". Flower described the fact that in Inia (and the same occurs
n Pontoporiay the palatines are separated from each other by the
ntervention of the vomer.     In this feature they resemble certain
.iphioids, SerardiuSj Oulodon ( = Mesoplodori) grayi, and Hypero-
don.     The true DolpKins also appear to  show the same inter-
ention of the vomer in a few cases.     There is nothing, therefore,
istinctive from the Delphinidae in this feature.
The existence of cartilaginous sternal ribs in Inia and
^lafanista shows affinity between these two genera and the
*hyseteridae. P&ntoporia is Dolphin-like in this particular, as
} is also in the mode of articulation of the ribs with the verte-
ral column. But this last matter has already been dealt with.
!he principal reason for placing Pontoporia with the other two
enera is the close resemblance which its skull bears to that of Inia.
The first geims of this family which will be noticed is Platanista.